This invention relates to a supporting apparatus adapted to support a semiconductor monocrystal rod grown by the floating zone (FZ) method.
The so-called FZ method is often adopted in a process for production of a semiconductor monocrystal rod. This method is effective in turning a polycrystal rod (such as a polycrystal silicon rod prepared through vapour-phase growth of silicon based on thermal decomposition or reduction of a substance containing silicon, or by simply casting silicon into a rod) into a highly-purified monocrystal rod. To achieve this effect, the FZ method employs a heating means such as an induction heating coil to heat a vertically held polycrystal rod. The heating means heats only a small portion of the rod into a narrow melted zone at a time, but as the vertically held rod is slowly lowered passing the heating means, the molten zone sweeps the whole length of the rod, starting at the joint with a seed, to eventually turn it into a pure monocrystal rod. The impurities are more apt to be solved in the melted zone than to stay in the solidifying portions, so that the melted zone collects the impurities as it sweeps the rod upward. Hence, the resultant monocrystal rod is free from impurities.
In this process of monocrystalization, the melted portion of the rod forms a so-called floating zone between the unmelted portions of the rod and is held therebetween by its surface tension and the electromagnetic force against gravity. As the floating zone travels upward in the rod accompanying the downward movement of the rod relative to the heating means, the portion which previously was a part of the floating zone solidifies to form a part of the monocrystal rod.
Incidentally, the FZ method utilizes a thin seed crystal to provide the starting source for the growth of the monocrystal rod. The polycrystal rod is suspended from above and its lower end is melted to merge with the upper tip of the seed crystal. As the monocrystal starts growing from the seed crystal, it forms a contracted thread-like portion before it begins growing into a conical portion. Although the polycrystal rod is suspended from above, the newly formed monocrystal portion of the rod, i.e. the portion below the floating zone, is supported via the thread-like portion and the thin seed crystal, so that as the monocrystal portion grows longer it becomes so heavy that even a slight vibration may cause the melt to drip and, in an extreme case, cause the thread-like portion or even the seed crystal to break. Hence, it is necessary to support the monocrystal portion of the rod by a means in addition to the seed crystal.
Several supporting apparatuses for this purpose have been proposed by, e.g. Japanese Provisional Patent Publication (Kokai) No. 52-6310, Japanese Patent Publication No. 57-50754, and Japanese Patent Publication No. 60-48479, but these apparatuses necessitate enlargement of the unit in which they are installed, and in terms of the stability of the monocrystal rod they support, they are yet unsatisfactory.